Sitting at Work Not Hazardous to Your Health

Action Points

Explain to interested patients that a systematic review failed to find that occupational sitting was associated with increased BMI, cancer, or cardiovascular disease.

Note that there was considerable heterogeneity among studies and that the investigators suggested that future studies quantify occupational sitting time and include measures of physical activity.

Adverse health outcomes including cancer and cardiovascular disease can't be blamed on prolonged occupational sitting, at least not until additional clarifying research is done, a systematic review concluded.

For instance, among 17 studies that examined a potential association between occupational sitting and cancer, only five (four case-control) found a higher risk, according to Jannique G.Z. van Uffelen, PhD, of the University of Queensland in Australia, and colleagues.

In contrast, ten prospective studies found no association between cancer and occupational sitting, and in two there actually was an increased risk of lung cancer in people who were active at work, the researchers reported online in the American Journal of Preventive Medicine.

Considerable evidence suggests that excessive sedentary behavior is associated with health risks, but most research in recent years has focused on sedentary leisure time activities, particularly television viewing.

However, because so many occupations today are predominantly sedentary, van Uffelen and colleagues undertook an examination of the evidence for prolonged sitting at work and various health risks.

They identified 43 studies that met their inclusion criteria, and rating them on a 15-point quality assessment scale yielded a median quality score of 12.

For the effects of prolonged sitting at work on body mass index (BMI), they found 12 studies. Nine were cross-sectional, two were prospective, and one reported both cross-sectional and prospective data.

Only five of the cross-sectional studies showed a positive association between sitting and higher BMI, and one found a negative association.

Furthermore, for BMI only one of the prospective studies found a positive association.

Eight studies (six prospective and two case-control) evaluated the association between cardiovascular disease and occupational sitting.

Four found an increased risk of myocardial infarction or coronary heart disease with prolonged sitting at work, but three showed no association and one found an increased cardiovascular risk with occupational activity.

Among four studies of diabetes risk and occupational sitting, one cross-sectional and two prospective studies found an increased risk with sitting, while an additional prospective study found no association.

Six prospective studies assessed mortality risk with occupational sitting, four of which found an increased risk.

However, one study found no association and one showed a decreased mortality risk for those with sedentary occupations.

In discussing their findings, van Uffelen and colleagues noted that the World Cancer Research Fund/American Institute for Cancer Research grades evidence on causality for health risks according to specified criteria.

The first two criteria, that there is evidence from more than one study type and from at least two independent cohort studies, were met only for cancer and cardiovascular disease in this study.

The next level of probable evidence, that there be no unexplained heterogeneity among studies, was not met for any of the outcomes.

Three additional criteria also must be met for evidence to be considered convincing.

The first of these additional criteria states that the quality of the studies must be sufficient to exclude the possibility that the association is a result of "random or systematic error," such as through confounding and selection bias.

The authors observed that, while the quality of studies in their review was generally good, few reported on the validity of sitting time measures. Future studies should do so and should also adjust for physical activity.

The next criterion calls for a dose-response, or plausible biological gradient, to support a causal relationship.

Most studies in this review did not quantify the time spent sitting, which may have contributed to the lack of an observed association with health risks. Future studies should include objective measures of both occupational and leisure-time sitting duration.

The final criterion requires evidence from human or animal studies that there is biological plausibility for the observed association and the health outcome in question.

The authors stated that emerging lines of evidence suggest that this is so.

"Physiologically, it has been suggested that the loss of local contractile stimulation induced through sitting leads to both the suppression of skeletal muscle lipoprotein lipase activity (which is necessary for triglyceride uptake and high-density lipoprotein cholesterol production)," they explained.

Sitting also can have negative behavioral effects, such as increased snacking.

Limitations of the study include the possibility that some studies may have been missed because of the lack of a standard term for occupational sitting.

The authors concluded that there is insufficient evidence at this time to draw firm conclusions as to the effects of occupational sitting on health risk.

This review was funded by Health Promotion Queensland.

The authors reported receiving grants from the Australian National Health and Medical Research Council, the Heart Foundation of Australia, and the Victorian Health Promotion Foundation.

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